Research on the structural optimization of China's multi-pillar pension system 并列题名：Research on the structural optimization of China's multi-pillar pension system Research on the structural optimization of regional media ecology Research on the structure optimization of grain industry Research on route selection of China's rural workers' pension system Research on the economic effects of China's new rural pension system Research on the spatial optimization of major cities in western China Research on the optimization of China's foreign trade structure Research on the optimization of local tax system Research on the governance structure optimization of physical education in Chinese universities